Polyester resins are often used in a variety of applications where stability against the deleterious effects of UV radiation found in sunlight is critical. These effects may concern the stability of the resin itself or the stability of the substance present on the other side. Of particular concern is the UV light at wavelengths between 320 and 390 nm, which are present in sunlight and which are being transmitted through standard PET.
One example of such need for protection are clear containers commonly used to store beverages, detergents, cosmetics and other products whose color or content (such as vitamins or flavor components) is sensitive to UV light.
Another example is fibers, used for example in outdoor applications such as woven signs, awnings, or outdoor garments that must provide skin protection to the wearer.
Other examples are clear extruded sheets used outdoors, for example as awnings, greenhouse roofs, advertising signs, etc.
Other examples yet are biaxially-oriented films. Biaxially oriented polyester films are often used in applications where stability to prolonged exposure to sunlight is critical. Such applications include weatherproof solar shell back sheets and clear window films.
In the case of solar cell back sheets, where clarity is not an issue and often undesirable, the most cost-effective was to impart UV resistance is by incorporating whitening agents such as titanium oxide or barium sulfate. However such films suffer from haze and are not suitable for production of films used in applications requiring good transparency, such as window films.
Such window films must often have very low haze in order to satisfy end-user applications. At the same time, they require ease in handling and processing and scratch resistance. This dual requirement is accomplished for example by incorporating particles, offering anti-block, slip, and anti-scratch characteristics but have a size below or within the range of visible light wavelength so as to prevent significant light diffraction that would result in haze. A very tangible benefit is the significant reduction of high angle sun haze, a factor in many solar applications.
Window films are often dyed or metalized (to an optical density that still leaves the film transparent to visible light) or have ceramic coated applied to convert incoming solar radiation to infrared radiation, which is then rejected back through the glass to the exterior.
The adhesive system used in window films rejects UV Radiation up to 380 nm. As UV is one of the main sources of fading, it can prolong the life of fixtures and fittings. Specialty UV Window Films are available that offer increased protection to 400 nm, through incorporation of high-performance UV absorbers, which also increase the shelf life of the film itself.
UV absorbers are compositions which absorb light in the wavelength area where PET is transparent to UV light and thus susceptible to damage by the energy absorbed. In doing so they consume the light energy themselves and thus it is not available for damaging the polyester.
UV absorbers are typically incorporated in the form of pre-compounded chip concentrates (“masterbatches”) in the manufacturing of polyester articles, such as bottles, fibers, and films. Such masterchips are produced during a separate step involving kneading the polymer pellets together with the UV absorber in a twin screw extruder which allows precise metering. This process adds additional cost. Furthermore, due to the thermal degradation high IV more expensive ship has to be used in forming the masterbatch.
A typical UV concentrate “masterchip” is formulated with a UVA content between 10-20% by weight
U.S. Pat. No. 4,617,374 describes the incorporation of a UV absorbing species belonging in the methane family and possessing a hydroxyl and a methyl ester end-group at the beginning of the esterification by a condensation reaction. U.S. Pat. No. 7,541,407 describes the incorporation of similar compounds after the esterification step and before the vacuum polycondensation step; the incorporation yields reported in the examples (25%, 44% and 77%) suggest that significant amount is lost due to volatility.